It's worded that it "uses a lighter mainspring than previous Ruger double-action 22 LR revolvers." Most rimfire revolvers have a heavier double action pull than centerfire models do, so I'm guessing this GP100 has a lighter trigger than either the SP101 or LCR in 22lr.
Good Point! Perhaps they use the standard 14 lb mainspring which would be lighter and due to leverage in the medium frame feel lighter than the small frames. Had hoped it might have been more exotic like a change in geometry inside or something.